My wife read a lot to our eldest son, and he taught himself to read before age 4 without any formal instruction. Early reading without formal instruction is one marker of giftedness. We entered him in kindergarten a year early, and as a 2nd grader his SCAT scores qualified him for the CTY courses in both reading and math.

So yes, I think giftedness can be identified in 3 and 4 year olds.

"To see what is in front of one's nose needs a constant struggle." - George Orwell